The present invention relates to a valve/actuator torque overload protector, and more particularly, to a valve/actuator torque overload protector having driving and driven members which are pre-set to transmit limited torque therebetween during rotary movement between the members.
Valve/actuator torque overload protectors are used in conjunction with valve/actuators of water transmission lines, for example, to prevent valve/actuator damage caused by applying too much operating torque to open or close the valve/actuators. In addition to water transmission lines, such devices may also be used in gas or other fluid transmission lines. Obviously, other industrial uses can be made of this type of valve or torque protector. Typically, such devices transmit torque only up to a pre-set amount, and they automatically disengage if the pre-set torque is exceeded. They are also designed to automatically re-set when the applied torque is below a pre-set amount.
As will be appreciated, valve/actuator torque overload protectors prevent valve/actuator damage which not only saves the cost of valve repair, but also the high cost of excavation, removal and replacement of the valve/actuators.
In the prior art, two drive configurations have been commonly employed between driving and driven members in valve/actuator torque overload protectors. One common prior art design has spherical balls in one of the members that operate in conjunction with conical detents in the other member. The other well known prior art design employs cylindrical rollers in one member with mating trough-like detents in the other member. Both of these design operate to allow relative rotation between the driving and driven members only until a pre-set torque is exceeded, in which case, the spherical balls or cylindrical rollers move out-of-register with the corresponding shaped detents to disengage the driving and driven members, and thereby prevent damage to the valve/actuator.
While these prior art designs have worked well for their intended purpose, the contact stress between the spherical ball or cylindrical roller and the mating detents or troughs, as they move out-of-register, causes distortion and wear of the devices, resulting in less precision, inconsistent repeatability, point contact focusing of forces, and consequent shorter product life. These contact stresses are encountered at the corner of the detent or trough, as a spherical ball or cylindrical roller moves out of the detent or trough.
Recognized stress analyst references, such as Formulas For Stress And Strain by Raymond J. Roark, Fifth Edition, McGraw Hill Book Company, 1984, show that under equal loads, the contact stress between a cylindrical roller with its length equal to it diameter and a flat surface is about one order of magnitude less than the contact stress between a spherical ball and flat surface.
Even though a cylindrical roller has less contact stress than a spherical ball, the cylindrical roller will always make point contact with the detent or trough corner as it rolls out of the detent during circular motion because the outer end of the cylindrical roller must travel a greater distance than a point on the cylindrical roller which is closer to the center of rotation of the driving and driven members. Thus, as the cylindrical roller exits the detent or trough, it always exits at an inclined angle to the detent or trough corner, thereby causing point contact with resulting high contact stress.
Other disadvantages in current valve/actuator torque overload protectors include the lack of a seal, the inability to adjust the trip torque, the requirement of disassembling the device to adjust the trip torque, and the necessity of complicated and expensive designs in prior art designs to achieve desired results.